also, there are 9 values in this field, in case that matters. (In the admin I have it set to display 10

This form is in Legacy mode, but I've tried in both legacy and non legacy, no difference. 

Also, I do not have either of these options checked

unchecked: show as dual listboxes in design/preview mode

unchecked: display dual listboxes vertically (html5 only)

I'm having a hard time visualizing what the script is trying to do. Can you add a screen print or two? 

Vickie

I don't know of a way to get it to a single list box. For the two list boxes. the unselected list has an id with '_LEFT' appended on it. I would remove any options that you don't want people to select from it. Here is code that I used to remove options from a possible list:

jQuerySBM("#"+jQuerySBM(GetFieldByName("TYPE_OF_WORK_COMPLETED")).attr("id")+"_LEFT").children("option").each(function(){var t = jQuerySBM(this).text(); if (t.indexOf('COTS')>-1){jQuerySBM(this).remove()}})

Note that if you need that option available again. I would only hide the option above instead of .remove().

For setting the list values, you can use the SetMultiListValues("LINE_ITEMS_TO_RENEW", ["Administration"])

This works perfectly for removing values. how would I hide / unhide values? as this is depending on another field, I may have several values to hide / show

I figured it out, using show() or hide() instead of remove()